.form-back-box {
  /* background-image: url('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1597320012153&di=8259d439b3045190b79b750eaea69d3f&imgtype=0&src=http%3A%2F%2Ffile02.16sucai.com%2Fd%2Ffile%2F2014%2F0807%2F247d1b1d3fc65f7516856244257f71ec.jpg'); */
  background-repeat: repeat;
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
}

.form-box {
  width: 500px;
  height: 300px;
  position: fixed;
  left: calc(50% - 250px);
  right: calc(50% - 250px);
  top: calc(50% - 150px);
  bottom: calc(50% - 150px);
}
